From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-99073][LOW] ipv4: stop exception dump when nexthop group shrinks Date: Mon, 26 Oct 2026 20:51:46 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-99073 X-AL-KERNEL-Priority: LOW X-AL-KERNEL-Severity: LOW X-AL-KERNEL-Base-Severity: LOW X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 0 X-AL-KERNEL-ActionableScore-Lower: unknown X-AL-KERNEL-Commit: f91e472821e32f58b03eb631e20a67157e2b928b List-Id: CVE: CVE-2026-99073 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: ipv4: stop exception dump when nexthop group shrinks Commit: f91e472821e32f58b03eb631e20a67157e2b928b Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f91e472821e32f58b03eb631e20a67157e2b928b Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99073 Analysis date: Mon, 26 Oct 2026 20:51:46 -0400 ActionableScore: 0 ActionableScore lower bound: unknown Actionable bucket: Ordinary Moderate Manual review required: NO Summary: A delegated network-management actor can race an IPv4 FIB dump with nexthop-group shrinkage and trigger a kernel NULL dereference. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99073 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99073 The original announcement does not normally provide a security severity estimate, CVSS assessment, or enough information to determine whether the reported kernel bug represents a practically relevant security issue. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. It combines an autonomous classifier with LLM-assisted technical analysis and a separate ActionableScore mechanism. The purpose of this report is to prioritize Linux kernel CVEs before manual review, identify cases that require prompt investigation, and support automatic closure of issues that are unlikely to have meaningful security impact. Published priority for this report: LOW Manual review required: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-99073 LOW https://www.kernelcve.org/list/?q=CVE-2026-99073 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=LOW RESULT=DIFF TEMP_DISTANCE_MERGE_SKIP=AEGIS_NO_POSITIVE_SEVERITY_EVIDENCE (final Legacy and Aegis differ; FINAL: LOW, Aegis: MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-362;**CWE-476;BEST CVSS score: '4.7';DESCR 'An IPv4 FIB exception dump can race with replacement of a nexthop group by a smaller group. fib_dump_info_fnhe uses an earlier path count as its loop bound while fib_info_nhc reloads the current group and can return NULL for an index removed by the concurrent replacement. Dereferencing nhc_flags then causes a NULL pointer dereference and can crash the kernel. For the CVSS the PR:L value reflects that reliable triggering normally requires a local actor able to initiate route dumps and control nexthop replacement through delegated CAP_NET_ADMIN. AC:H reflects the required timing window. The issue is not directly reachable through ordinary remote IPv4 packets and there is no supported UAF, out of bounds access, information leak, or write primitive beyond the kernel crash. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 0 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE to LOW because the ActionableScore threshold rule was applied. final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic and the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 and KPANIC_UNSUPPORTED_REMOVED and V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED. The independent weaker Aegis shadow evaluated CVSS 1.9 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0) MAYBE REMOTE SIMPLEFIX NETWORK DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 KPANIC_UNSUPPORTED_REMOVED V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED - - checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=0 PrimitiveClass=CLASS_C RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=UNKNOWN ImportantCandidate=NO ManualReview=NO AutoDowngradeAllowed=YES RequiresHostUID0=NO AuthorizationDecisionBypass=NO ProtectedResourceImpact=NONE B4PendingReview=NO B4PendingConfirmed=NO 1. ActionableScore * Raw base score: 0. * Conservative score: 0; no floor or cap applied. * Raw paranoid score: 0. * Paranoid score: 0; no technically supported stronger primitive. * Final recommended bucket: **Ordinary Moderate**:: 2. Signal breakdown * +1 Reliable kernel crash: concurrent group shrink can make `fib_info_nhc()` return NULL, followed by dereference of `nhc->nhc_flags`. * -1 Reduced/delegated privilege: supplied evidence grounds the complete trigger in an actor with delegated `CAP_NET_ADMIN`, below host-admin-equivalent privilege. * No race penalty: the race window exists, but repeated dump/replacement attempts are actor-driven; the evidence does not establish an uncontrollable one-shot race. * No UAF, OOB access, write primitive, disclosure, authorization bypass, or integrity impact is shown. * NEW52 B5-R does not apply: there is no retry-accumulated heap OOB write or later consumption of corrupted structured state. * Extra availability is not awarded because it would duplicate the same NULL-dereference crash. 3. Reachability analysis A local actor can initiate the FIB dump while replacing a nexthop group with fewer paths. Replacement requires delegated `CAP_NET_ADMIN`; ordinary remote packets do not reach this operation. Below-host-admin reachability is supported, but the evidence does not identify the exact capability namespace or prove that an attacker-created user/net namespace supplies the complete trigger, so `NamespaceScopedPrivilege` remains unknown. No special hardware or boot option is required. 4. Primitive and severity reading The primitive is a race-induced NULL pointer dereference, not stale post-lifetime use or memory corruption. The fix stops iteration when the independently reloaded path is absent. The shown effect is kernel crash/DoS; useful effects past the crash are unsupported. This is CLASS_C, not an Important candidate, and automatic downgrade is permitted. 5. One-sentence report phrase A delegated network-management actor can race an IPv4 FIB dump with nexthop-group shrinkage and trigger a kernel NULL dereference. 6. Manual review advice NO MANUAL CHECK NEEDED The patch and call-path evidence support only a constrained crash primitive. 7. Call-site confidence Call-site confidence: high ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: ipv4: stop exception dump when nexthop group shrinks Commit: f91e472821e32f58b03eb631e20a67157e2b928b Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f91e472821e32f58b03eb631e20a67157e2b928b Commit description: Commit 4ce5dc9 ("inet: switch inet_dump_fib() to RCU protection") allowed IPv4 FIB dumps to run concurrently with nexthop replacement. fib_dump_info_fnhe() uses fib_info_num_path() as the loop bound, then fib_info_nhc() independently reloads nh->nh_grp. If RTM_NEWNEXTHOP replaces a group with fewer paths between those loads, fib_info_nhc() returns NULL and the dump dereferences it through nhc_flags. Stop the walk when the indexed path is absent. Nexthop groups are dense, so the current snapshot has no later path. Fixes: 4ce5dc9 ("inet: switch inet_dump_fib() to RCU protection") Reported-by: Ido Schimmel Link: https://lore.kernel.org/netdev/20261001170513.GA1657889@shredder/ Cc: stable@vger.kernel.org Signed-off-by: Daehyeon Ko <4ncienth@gmail.com> Reviewed-by: Ido Schimmel Link: https://patch.msgid.link/5bb7bd97411eb9db454464bf449107395e760f26.1790915964.git.4ncienth@gmail.com Signed-off-by: Jakub Kicinski Signed-off-by: Greg Kroah-Hartman Changed files: net/ipv4/route.c Diff excerpt: Not included in this email. See the upstream URL for the full patch. Full patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f91e472821e32f58b03eb631e20a67157e2b928b ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-99073 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99073 The original CVE announcement normally does not include a security-level estimate. In particular, it may not contain a CVSS assessment, an impact level, or enough information to determine whether the reported bug is a practically relevant security issue. One purpose of this parallel CVE list is to provide that missing technical and prioritization information. The original goal of the AL-KERNEL project was to prioritize Linux kernel CVE analysis automatically before manual review. The system can also help identify non-security issues that may be suitable for automatic closure. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. The first analysis stage combines an autonomous classifier with additional LLM-based analysis. The autonomous classifier runs locally on a CPU and is based on a backpropagation neural network. Together, these mechanisms produce a technical vulnerability description, identify likely weakness types, estimate CVSS severity, and provide input for ActionableScore. Two CVSS estimates are retained because incomplete kernel vulnerability information often permits more than one defensible interpretation: Conservative CVSS vector: Not available The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 4.7 The conservative vector represents a lower-impact interpretation. The Best/paranoid vector intentionally represents a plausible upper-bound interpretation and should not automatically be treated as demonstrated real-world impact. CVSS may also need to be adjusted for a particular Linux deployment, because actual reachability, privileges, enabled kernel configuration, hardware, namespaces, exposed device nodes, and other environmental conditions can differ significantly between systems. A separate ActionableScore mechanism evaluates practical remediation urgency. Its analysis may include reachability, attack prerequisites, subsystem exposure, memory-corruption characteristics, denial-of-service reliability, and possible confidentiality, integrity, or privilege-escalation impact. Conservative ActionableScore: unknown Paranoid ActionableScore: 0 The final base severity is taken directly from the second tab-separated field of the AL-KERNEL classification result. ActionableScore does not replace or independently override that final AL-KERNEL decision, and if ActionableScore adjusted impact level of ALKERNEL, then you would see self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7. For an AL-KERNEL result of MODERATE, this report uses the following additional presentation split: ActionableScore below 5 -> MODERATE REGULAR ActionableScore 5 or more -> MODERATE 7.0 The distinction between MODERATE REGULAR and MODERATE 7.0 makes it possible to identify Moderate issues that should receive manual analysis and fixes before lower-priority MODERATE REGULAR issues. In many cases, MODERATE REGULAR fixes may wait for a later rebase or routine update. There is one override in which MODERATE REGULAR becomes MODERATE 7.0 even when the ActionableScore is below 5. When the AL-KERNEL result contains the KPANIC flag, a MODERATE result is always presented as MODERATE 7.0. The KPANIC flag selected with few regexps without usage of AI at all, so it helps to detect cases when Kernel Crash happens and similar (to filter False-Negative results from the LLM usage). KPANIC indicates that a reliable kernel crash, kernel panic, or similarly serious kernel availability impact was identified by the classification workflow. AL-KERNEL base severity for this report: LOW KPANIC detected for this report: NO Published priority for this report (same as in Subject): LOW These results are intended to support engineering triage. They are machine-generated estimates, and cases marked for manual review should be validated by a human security engineer before final disposition. For more info read docs linked from here: https://kernelcve.org/ (and you can submit you own patch there to generate such a report for non-existant CVE-id yet). Note that in many cases this AI tool selects higher severity, than real is (means you can expect Importants instead of Moderate 7.0 or Moderates 7.0 instead of regular Moderates). If you see such cases, please use reply email interface to add additional manual analyses info to this particular CVE. And please, please, let me know when you see Lows instead of Importants or Important instead of Low (because particular for such cases I need to tune this AI tool to make it better for this one and next similar). My contact email for such notifications is alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).